Now there exists an increasing demand and promising trend to integrate multimedia services with Wireless Sensor Networks (WSNs). Hence, amount of various multimedia data, such as an image, audio, video, etc., will grow larger and larger and far exceed that of traditional scalar measurement data. Supporting multimedia applications over WSNs requires a low latency and high communication efficiency at the same time, which brings particular challenges to the resource (e.g., power, memory, and so on) limited WSNs.
It is known that the IEEE 802.15.4 WPAN standard is specifically designed to achieve a low-cost and low-power wireless connectivity among resource limited devices. For example, the IEEE 802.15.4 Medium Access Control (MAC) has presented commonly recognized solutions for implementing a low duty cycle in WPANs. In order to fulfill various application scenarios, the IEEE 802.15.4 provides two operational modes, i.e., Beacon-enabled and Non Beacon-enabled modes, for choice. The Beacon-enabled mode may save energy but limit data throughput by adopting a RF sleep mechanism. In contrast, the Non Beacon-enabled mode may provide higher data throughput but consume significant energy due to continuous RF idle listening.
For scalar sensor devices, energy efficiency is a primary concern because scalar measurement traffic is generated at low rates. Therefore, the Beacon-enabled mode would be preferable to the Non Beacon-enabled mode for this case. However, for multimedia sensor devices, the result would be reversed because multimedia data traffic requires higher throughput which is confined by the Beacon-enabled mode. Unfortunately, WPAN operational mode is usually determined and configured at an initialization stage of the network and only one of the modes may be supported at each time. For these reasons, it is difficult for IEEE 802.15.4 to achieve both high energy efficiency and data throughput for the WPANs which would carry multimedia and scalar data traffic at the same time. In addition, when many multimedia sensor devices enter or leave the network after the initialization stage, it is so hard for the network to be adaptive to such variations.
The existing solution to the foregoing problem is the Traffic and Energy Aware IEEE 802.15.4 (TEA-15.4) scheme. In the proposed TEA-15.4, a PAN coordinator can adaptively adjust an active period according to data traffic information of the associated devices in the Beacon-enabled mode. TEA-15.4 employs two mechanisms to inform the PAN coordinator of traffic information. The first one is based upon the Arbitrary Traffic Signal (ATS) (hereinafter as “ATS scheme”) and the second one utilizes the Traffic Time-Out (TTO) (hereinafter as “TTO scheme”). The ATS scheme is designed to detect an arbitrary traffic frame or its collision signal that indicates the existence of the data traffic, whereas the TTO scheme utilizes a time-out mechanism to detect the data traffic information of the associated devices. Both mechanisms are periodically performed during sentinel duration, i.e., a special epoch as decided by the PAN coordinator for detecting the traffic information.
The major drawbacks of the ATS scheme are that: 1) additional 40-symbol ATS sentinel frames are introduced and transmitted periodically within one Beacon interval. These frames would lead to additional traffic and energy consumption; 2) the PAN coordinator can make rather rough but not suitable active duration adjustment because the PAN coordinator is unable to obtain, from the ATS frames, sufficient traffic information of overall devices.
The major drawbacks of the TTO scheme are that: 1) TTO introduces periodic 620-symbol sentinel duration within each Beacon interval, so that sensor devices or nodes have to stay much longer in the active mode so as to detect traffic presence, resulting in higher energy consumption; 2) For low data traffic scenarios, TTO wastes too much time and energy in RF idle listening and no adaptation is included in this scheme.